1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance imaging apparatus to produce tomograms in arbitrarily oriented planes, as well as to a circuit for operating a magnetic resonance imaging apparatus wherein one gradient coil is operated in a resonant circuit.
2. Description of the Prior Art
a magnetic resonance imaging apparatus in which one gradient coil is operated in a resonant circuit is used, for example in the echo planar imaging (EPI) method, as described, for example, in European Application 0 076 054. Typically in the EPI method, the raw data for the construction of a complete tomogram are acquired after a single excitation of the examination subject. The high switching speed of, in particular, the read-out gradient which is necessary for this purpose is isually achieved by operating the gradient coil which generates the read-out gradient in a resonant circuit.
Image acquisition in magnetic resonance tomography usually ensues slice-by-slice, with a slice of the examination subject being first selectively excited, with only signals from this slice then being received. The image acquired from these signals then represents a tomogram in the selected slice. The production of tomograms in arbitrarily rotated planes is advantageous for many applications, so that the information relevant for the examination is contained in one sectional plane. For this purpose, it is necessary that slices which are arbitrarily spatially oriented must be excited and read out. This can be achieved using traditional gradients, i.e., gradients which are not switched resonantly, by generating resulting gradients having an arbitrary spatial orientation by simultaneously activating a plurality of gradient coils, and by a corresponding dimensioning of the "physical" gradient, i.e., the gradient produced by the individual gradient coils.
For distinction from the aforementioned physical gradients, the resulting gradients shall be referred to herein as "logic" gradients.
The employment of tilted slices has been proposed for use in the EPI method in "40 Millisecond Instant Long Axis Heart Imaging," Weisskopf et al., Book of Abstracts, SMRM 1990, page 123. Only a tilting around the axis of the frequency-coding gradient, however, can be achieved by controlling the slice selection gradient and the phase-coding gradient, as proposed in this article. A combination of all three physical gradients, i.e., including the frequency-coding gradient, is necessary for arbitrary slice orientation. For gradients which are generated by means of a resonant circuit, however, difficulties are encountered using conventional techniques, as are described below with reference to FIGS. 1 through 7.
As is known, a topical resolution of the nuclear magnetic resonance signals in a magnetic resonance imaging apparatus is achieved by superimposing a magnetic field gradient on a uniform, static fundamental field, on the order of magnitude of 1 Tesla. The principles of such imaging are set forth in the article "NMR-Imaging Techniques and Applications: A Review", Bottomly, Review of Scientific Instrument, 53(9), September, 1982pages 1319-1337.
For topical resolution in three dimensions, magnetic field gradients in three directions, preferably orthogonally disposed, must be produced. The x, y, z axes of a Cartesian coordinate system are shown in FIG. 1 to indicate the direction of the physical gradients G.sub.x, G.sub.y and G.sub.z produced by the gradient coils in the system of FIG. 1. The gradient coils 2, in the form of saddle coils, generate a physical magnetic field gradient G.sub.y in the y-direction. A substantially constant magnetic field gradient G.sub.y in the y-direction is generated within a spherical examination volume 4 by the conductor sections 2a. Due to their greater distance from the examination volume 4, the return conductors produce only negligible magnetic field components in the examination volume 4.
The gradient coils for generating the physical magnetic field gradients in the x-direction are constructed identically to the gradient coils 2 for the y-direction magnetic field gradient, but are rotated 90.degree. in azimuthal direction of the cylindrical carrier 1. For clarity, these gradient coils are therefore not shown in FIG. 1.
The gradient coils 3 for generating the physical magnetic field gradient in z-direction are annular, and are arranged symmetrically relative to the center point of the examination volume 4. The two individual coils 3a and 3b respectively carry current flowing in opposite directions, as indicated in FIG. 1, so as to produce a magnetic field gradient in z-direction.
A typical pulse sequence of the EPI method is set forth below.
At the beginning of the pulse sequence, the examination subject is subjected to a radio-frequency excitation pulse RF, shown in FIG. 2, in the presence of the positive portion SS+ of a slice-selection gradient SS in the z-direction, as shown in FIG. 3. Nuclear spins in a slice of the examination subject are thus excited. Subsequently, the direction of the slice selection gradient is inverted, so that the negative portion SS- of the gradient SS cancels the dephasing of the nuclear spins caused by the positive portion SS+ of the gradient SS.
After the excitation, a phase-coding gradient PC, shown in FIG. 4, or PC' shown in FIG. 5, is activated in the y-direction, and a read-out gradient RO, shown in Figure 6, is activated in x-direction. The read-out gradient consists of a pre-pulse ROV and a sequence of sub-pulses RO of alternating polarity.
Due to the alternating polarity of the read-out gradient RO, the nuclear spins are alternatingly dephased and rephased, so that a sequence of signals S arises, as shown in FIG. 7. After a single excitation with the ratio-frequency pulse RF, so many signals are acquired that the entire Fourier K-space is scanned, i.e., the data obtained with a single pulse RF are sufficient for reconstruction of the complete tomogram.
A phase coding is also implemented in addition to the read-out gradient RO which effects the frequency coding. Two possibilities for such phase coding are shown. In the exemplary embodiment of FIG. 4, the phase-coding gradient PC is constantly activated during the sequence. The phase of the nuclear spins thus continuously increases during the sequence. In the exemplary embodiment of FIG. 5, a brief phase-coding pulse PC' is activated at each change in the operational sign of the read-out gradient RO, so that the phase relation of the nuclear spins increases step-by-step over the pulse sequence. In both versions, the actual phase-coding pulses PC or PC' are preceded by respective pre-phasing pulses PCV or PCV'.
The nuclear magnetic resonance signal S arises under each pulse RO of the read-out gradient, the signal S being sampled in the time domain at defined times. The sampled signals are digitized and the numerical values acquired in this manner are entered in a raw data matrix. The raw data matrix can be considered a measured data space, and is thus a measured data plane in the two-dimensional case described in the exemplary embodiment. This measured data space is known to those in the art as "K-space."
The information regarding the spatial origin of the signal contributions S needed for the imaging is coded in the phase factors, the relationship between the locus space (i.e., the image) and the K-spaced existing mathematically via a two-dimensional Fourier transformation, as expressed by the following relationship. EQU S(K.sub.x,K.sub.y)=.intg..intg..rho.(x,y)exp[i(K.sub.x x+K.sub.y y)]dxdy,
wherein ##EQU1## .rho.=nuclear magnetic density, .gamma.=gyromagnetic ratio,
G.sub.x (t')=momentary value of the read-out gradient RO, and PA1 G.sub.y (t)=momentary value of the phase-coding gradient PC.
When, as shown in the aforementioned example, the slice gradient SS is produced only by means of a gradient coil (i.e., the slice selection gradient SS is a physical gradient), such as the physical gradient G.sub.z in the example, the position or attitude of the slice, and thus of the tomogram which is acquired, is limited to slices which lie perpendicularly relative to the z-axis. The phase-coding gradient PC and the read-out gradient RO must always lie orthogonally relative to each other and relative to the slice-selection gradient SS. This is most simply achieved by producing the phase-coding gradient PC exclusively with either one of the physical gradients G.sub.y or G.sub.y2/3 and the read-out gradient RO is produced exclusively with the physical gradient G.sub.x.
In conventional methods which do not use resonant gradients, it is nonetheless possible to produce arbitrarily spatially oriented slices or sections by forming logic gradients using the components from the physical gradients G.sub.x, G.sub.y and G.sub.z to generate the slice selection gradient SS, the phase-coding gradient PC and the read-out gradient RO. In order to produce such a resulting gradient (referred to below as a "logic" gradient) having an arbitrary spatial orientation, all of the physical gradients G.sub.x, G.sub.y and G.sub.z must be capable of being simultaneously activated with respectively selected amplitudes. As set forth in FIGS. 3 through 6, however, this is not possible given resonantly switched gradients without undertaking further steps.
As already mentioned, the arbitrary spatial orientation of a logic gradient requires the possibility of simultaneously activating the three physical gradients G.sub.x, G.sub.y and G.sub.z. This means that all three gradient coils would have to be selected with a direct current for forming a logic phase-coding gradient PC in the form of FIG. 4. At the same time, however, the three gradient coils would have to be resonantly operated to form the read-out gradient RO in the form of FIG. 6. It is impossible, however, to simultaneously operate a gradient coil in a resonant circuit and charge it with a direct current.
If a logic gradient PC' in the form of FIG. 5 is used, all of the gradient coils could be operated in resonance. The resonant frequency, however, for producing the logic phase-coding gradient PC' would have to be a different frequency than that for producing the logic read-out gradient RO, because the phase-coding pulses PC' are significantly shorter than the read-out pulses RO. It is not possible to generate both pulses RO and PC' with different frequencies at the same time.
In the aforementioned Weisskopf et al. article, the production of tilted slices is discussed, with a phase-coding gradient corresponding to FIG. 5 being used. In the article, however, it is proposed that only a conventional y-gradient and a conventional z-gradient, (i.e., gradients not produced by resonance) are combined, which is possible without problems. A frequency-coding gradient (or a read-out gradient) produced by means of resonance is not involved in the combination discussed in the article, so that the above-described difficulties are not present. Using the techniques disclosed in the Weisskopf article, however, an arbitrary slice position or attitude cannot be achieved, because the gradient combination employs only two physical gradients.